1. Field
The present disclosure relates generally to aircraft and, in particular, to noise inside an aircraft. Still more particularly, the present disclosure relates to a method and apparatus for reducing noise inside an aircraft.
2. Background
Sound generated by an aircraft is often unwanted. Sound is an acoustical wave that is an oscillation of pressure transmitted through a medium such as a solid, a liquid, or a gas. Sound typically has one or more frequencies within a range that can be heard by a person. Unwanted sound is referred to as noise. For example, noise may be generated while the aircraft is on the ground, taxiing, taking off, ascending, during level flight, descending, landing, and during other phases of flight.
The noise generated inside an aircraft may come from various sources. For example, an aircraft may generate aerodynamic noise. Aerodynamic noise is noise that arises from airflow around the fuselage of the aircraft, the wings, control surfaces, and other surfaces of the aircraft. This type of noise typically increases with aircraft speed. Aerodynamic noise also may occur with the operation of parts of an aircraft such as rotors on a helicopter and blades on a propeller of an airplane.
Other types of noise may include engine and mechanical noise that is caused by the operation of an engine, gear box, transmission, and other components within or outside the aircraft. Noise may also occur from various systems such as environmental systems in an auxiliary power unit and other systems that may operate inside the aircraft.
The noise generated by the aircraft and from other sources external to the aircraft may be undesirable to people in the environment outside of the aircraft as well as passengers and crew members inside the aircraft.
These and other types of noises generated by an aircraft may be reduced by different mechanisms. These mechanisms may include, for example, active mechanisms, passive mechanisms, or a combination of active mechanisms and passive mechanisms. Active mechanisms may use elements such as speakers, actuators, microphones, and other components to generate an out-of-phase signal to cancel the noise. A passive mechanism may include devices such as absorbers, barriers, mufflers, silencers, and other components.
Currently, newer types of structures are being used in the construction of aircraft. These structures may be, for example, composite structures that are lighter in weight and have a higher stiffness than previously used metal structures. These types of structures are more efficient in radiating noise than metal structures. As a result, the control of noise within the aircraft may become more challenging with the use of composite materials in the aircraft.
Currently used noise reduction systems for reducing noise may be enhanced to take into account the increased noise that may be generated by these composite structures. However, technical challenges are present with regard to improving noise reduction systems for aircraft. These challenges include taking into account weight, cost, complexity, and performance of the noise reduction systems. In particular, improving the noise reduction capabilities of currently available noise reduction systems may result in an undesired weight and cost for those systems.
For example, currently used systems may include elements such as damping, fiberglass blankets, acoustic foam, trim panels, isolators, and other components. As an example, damping tiles may be placed onto structural panels to absorb vibration energy. Fiberglass blankets, acoustic foam layers, or both fiberglass blankets and acoustic foam layers also may be added to absorb noise that may radiate from these panels. In other cases, existing fiberglass blankets and viscoelastic damping tiles may be replaced with thicker versions.
These solutions may be less efficient, more costly, and add more weight than desired when redesigning noise reduction systems to take into account increases in noise that may be caused by composite structures. Therefore, it would be desirable to have a method and apparatus that takes into account at least some of the issues discussed above, as well as other possible issues.